Got repeatedly several of the below alerts on our production systems. .
TNEX DUMP.DATA.SET.Dyymmdd.Ttttttt CA MAT SRB Diagnostic dump
TNEX DUMP.DATA.SET.Dyymmdd.Ttttttt CA MAT SRB Diagnostic dump
TNEX DUMP.DATA.SET.Dyymmdd.Ttttttt CA MAT SRB Diagnostic dump
What is happening is that 64 bit addressing in IMS control blocks are causing the abends.
In MAT v11 this is not available, however it is supported by MAT v12.
Try the same monitors in your MAT v12 LPAR and see if they are resolved there.