Removing sequence works wrong with collapsing mode in MSA

Scenario to reproduce:

1. Open "data/samples/CLUSTALW/COI.aln".
2. Toggle the collapsing mode.
   Expected state: the only collapsed group is headed with the sequence "Mecopoda_elongata_Ishigaki_J".
3. Remove the first sequence.
   Expected state: the only collapsed group is headed with the sequence "Mecopoda_elongata_Ishigaki_J".
   Current state: collapsing group is headed with the sequence "Mecopoda_elongata_Sumatra". Note, that this group is incorrect.
4. Repeat step 3 four times
   Current state: SAFE_POINT triggered