Reimporting static mesh doesn't respect changed lightmap settings

Licensee reports change of behaviour between UE4 and UE5 when reimporting a mesh after changing its lightmap build settings.

Change of behaviour occurred in 17404767, which was to fix a similar problem.

Please see the UDN thread for more details.

Import the attached FBX, using one of the two below methods:

Method 1:

• Do NOT turn on UV lightmap gen in the initial import window
• In SM editor, should see base UVs in [0], junk islands in [1]
• In LOD0 build settings, toggle on generate lightmap UVs, set source channel to [1], set dest channel to [1], click apply.
• Should see packed UVs.
• Select reimport: Current behaviour is everything gets reset; desired behaviour is leave the generated lightmaps alone.

Method 2:

• If user turns ON initial lightmap gen, desired behaviour is to go ahead and pack the lightmaps into channel [2].
• Doing so correctly yields initial UVs in [0] and [1], and new lightmap UVs in [2].
• Reimport leaves everything correct.
• Manually changing the maps from [0] -> [2] to [1] -> [1] in the build settings, deletes channel [2] and properly packs channel [1] as expected.
• Desired behaviour is hitting reimport leaves everything correct, current behaviour reverts UVs back to initial state.