Remembering that i am in Australia and not 100% up on the different cakes and fillings others use..... so in saying that....I never fill a frozen cake - cold yes, but never frozen.
As the cake thaws the added moisture from the *frozen* cake can contribute to how your filling and crumb coat either stays or decides to fall/slide off.
The added moisture needs to escape (so to speak) and by being locked in due to the filling and crumb coat i am thinking that this is why some are finding air bubbles in thier filling.
I would suggest (and its only my thoughts) that the cake be allowed to thaw before filling.
This is differrent to a cold cake.
Something that i do do is....
After my cake is baked - i tort it into its layers...
Then stacking it back up again with freezer bags in between each layer.
Freezer bags work a treat as they make seperating the layers so easy when need be.
When i take my frozen cake from the freezer DURING THE DAY, i unwrap the glad wrap - seperate the layers - and after about 15 minutes my layers are ready to fillled and then stacked again.
IF i take my frozen cake out of the freezer at night time before i go to bed, i just leave it as a whole cake on the bench..... then in the morning - upwrap and seperate the layers.
After filling my cake and stacking the layers back up again - i lay sheets of baking paper on top - then place my extrememly heavy bread board on top - so as to help with the settling process.
I leave my cake like this for at least 3 hours.
Then i do my outter layer/crumb coat - which is ganache.
After this process has been done, i again place baking paper on top and put the extremely heavy bread board back on top - I then leave my cake for at least 6 hours - mostly oivernight.
As ganache does not have to be refridgerated - i am safe in doing this.
I understand that many use perishable fillings - and that thier *weighted cake* would need to go into a fridg.Please...this is how I do it - i am not saying this is how it must be done....just adding a suggestion to this thread....it may help some.