Fix very low muon ID in `forward_then_matching`

The isMuon efficiency is very low in the forward_then_matching sequence compared to the other ones. The cause needs to be found and fixed. Summary of the current status:

• SciFiv6_upgrade_DC19_01_Bs2PhiPhiMD_retinacluster_v1_newLHCbID_v2_new_UT_geometry_hlt1_pp_forward_then_matching_no_gec_validation_a5000.txt: 29.33% isMuon efficiency (all muons)
• SciFiv6_upgrade_DC19_01_Bs2PhiPhiMD_retinacluster_v1_newLHCbID_v2_new_UT_geometry_hlt1_pp_no_ut_validation_a5000.txt: 92%
• SciFiv6_upgrade_DC19_01_Bs2PhiPhiMD_retinacluster_v1_newLHCbID_v2_new_UT_geometry_hlt1_pp_matching_validation_a5000.txt: 89.91%

This is currently blocking the HLT1 BW division \cc @cofitzpa @tevans @jhorswil