InDesign 19.3: Items Jumping to Front Despite Being Sent to Back and Locked

Anyone else having this issue:

My team are currently having a glitch where items that have been sent to back are jumping back to the front even if the item is locked. There doesn't seem to be a pattern to this as this is happening while working elsewhere in the documents and if often only noticed once it has been PDFd. It is effecting images/text boxes and solid shapes.

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