Slitherine

It says here in 1.10 change log (Generate Tiles command no longer overwrites any tiles set manually by the designer. It is possible to generate tiles on a per-layer basis.)

Editor

More advanced scripting. Now it is possible to trigger not only single actions but also scripts which can consist of several actions. Each script can run one or several times, and also skip a few runs before it gets executed. Also, it is now possible to run scripts not only at the end of a turn, but immediately when the required condition is met (e. g. you capture a city - this triggers an event), and also at the beginning/end of axis/allies half-turn. There are new types of actions (display message, center on hex, mark or unmark certain hex as victory objective) and conditions. Setting and checking tags allow scripts to exchange information - a certain trigger could depend on one or several scripts running prior to its execution etc.
A number of new AI orders. It is possible to give the units to move to a certain hex via a given trajectory, or to move to one of several hexes randomly; patrol area in different ways, or disembark on one of several given hexes randomly. Also, the AI action allows to assign all these new AI orders based on a certain trigger.
Unit placement randomization. Both preplaced units and reinforcements can now appear at random location - within a given radius from their original hex, or in a given zone. This allows to do pretty tricky randomizations: for example, reinforcements could come from any location on the border of the map.
Heroes configuration. Now it is possible to assign any heroes to any units on the map.
Exit zones. Available via Special Hexes layer, separately for Axis and Allies. A unit which enters such a hex is removed from the map and is placed in Reserve. It is also possible to analyze units in reserve using map condition with "Where" set to "Reserve".
It is now possible to set max strength on a per-unit individual basis.
It is now possible to have several different transports of the same type in the same scenario (e. g. normal air transports and gliders at the same time). They are configured via Transports tab in Scenario Params.
Theatre setting. Any scenario can be set in Europe or Africa. Depending on theatre setting the units will use appropriate camouflage.
In addition to cloudy weather, rain and snow, it is not possible to specify probability of sandstorms in a given scenario.
Share flags setting. If set, units of any nation can be purchased next to flags belonging to allied nations (e. g. german units near italian falgs if germany and italy belong to the same side).
"Special" check box in unit params, allowing to mark mission-critical units (the ones you need to protect, escort, destroy) with a different strength plate.
Generate Tiles command no longer overwrites any tiles set manually by the designer. It is possible to generate tiles on a per-layer basis.
It is possible to change unit type without losing its stats - just select new type and click on the unit twice (first click will select the unit, second will change the type).
Units should no longer lose transports after editing unit params.


I lay some new terrain then generate tiles under tools and they still go missing?
Help!

Generate tiles is a complete regeneration.

If you want to generate the current layer use that option.

Also, hit the space bar and generate tiles when the space bar is on.

I think your problem is overlays. Overlays do not change the tiles. It just places eye candy on top of the existing tile.

When using eye candy, you have to use the generate current layer option.

Under edit terrain trying to generate new terrain tiles using both space bar on and off/ generate tiles still wipes other manually placed overlays.
There should be a option that says save current placed overlays no matter what unless i specify otherwise.

What version of the Editor are you using? In the latest versions Tools, Generate Tiles (Current Layer) will do just the current layer (!!)

I think his problem is;
he is using eye candy (overlays) to generate tiles in place of terrain, generate tiles.