In Microsoft Project (MSP) intermittently dependencies (predecessors and successors) are missing from the project. This is noticed intermittently if you (or another user) open the project from the Modern User Experience (UX).
This is caused by DE59052 noted in KB: New UX allows locked project to be opened Read-Write in MSP/OWB
Release: 15.8, 15.8.1, 15.9, 15.9.1, 15.9.2
Clarity Microsoft Project (MSP) Integration
This issue is fixed in 15.9.3.
The avoid this issue until you are able to upgrade, ensure the project is not locked before opening a project from the Modern UX. (This can be done in versions prior to 15.9.3 by checking the lock status of the project in Classic Clarity)
To restore any lost dependencies (if a recent copy of the project exists in a non production environment):