When a bo4 has height restrictions, if the initial spot that the bo4 checks for viability is outside the height range the log will report the following:

Branching BO4 icecrystal2 could not be spawned in minimum configuration (isRequiredBranch branches only).

In this case the bo4 is only 3x3 chunks in this case and has no 'checks' other than height range and has no random/optional branches.

Once this happens the bo4 will never place even in viable spots anywhere in the world - but if another (new) world is generated and the height check is passed (because biome height is variable) then the bo4 will generate fine within its height constraints.

Here is the biome with height restrictions on the ice crystals:

Here is the biome (same spot, same seed) without height restrictions on the crystals:

The key point here is that the trees have the same max height limit as the crystals in the first image, so both should be spawning.

Thanks for reporting, looks like height checks for the structure's start chunk were being done when calculating minimumsize, which ofc they shouldn't have been. So when calculating the minimumsize when a customstructure was first being plotted, if the height checks failed, the structure wouldn't be allowed to spawn anywhere in the biome.

Should be fixed for v8.4_r1, minimumsize is calculated without performing starting chunk height checks.