Thanks @Michael_Schmid - I guess this is related to the lack of proper handling indexes (storing) withing the backup. Also mentioned here for the other index - PBM restore failure -- missing required partialFilterExpression on the creates
Might be related to changing our implementation from mongodump to our own. We’ll try to fix it withing next version in 2.9.0. For that time I suggest to use 2.6.0 verison.