While merging entries were untying

[color=#008888]I have a question:
I have 2 account record and they are linked to other records in other modules (by simple relate field).
Now I need to merge this 2 account records into one record. I do “Merge”. And now I have one Merged account record, it’s OK. But As a result, I see that related records are lost and not merged into merged account. Why related records were lost?
Account have simple relate field to link records. Maybe I need use “relationship” between modules, not “relate filed”

Thank you in advance

SuiteCRM ver 7.7.5