I had similar-ish this weekend but it was more complicated than that. Suddenly towards the end of the day the card-light would keep blinking. Everything seemed to work normal, it'd just blink like crazy. Also it prevented me from adding stars to the pics ("function not available at the moment" or error message something like that)
I had both CFExpress and SD cards in, but saving pics to CFExpress only.
Things I tried:
-remove CFE -> still blinking
-remove SD (CFE in) -> Still blinking
-remove both -> no blink
-both card back -> blink
-swap new CFE card -> blink
-put the CFE in another body (R5) -> R5 blinking now
Also rebooted (and removed battery) couple times but always kept blinking.
So it was really strange that using new CFE card it was still blinking, but moving the "bad suspect" CFE card to another body also made that body to blink.
End of the day I removed battery for some >5 minutes and put in new CFE card and it stopped blinking. I feel there was something related to the card (or file stored in the card) that made the R3 FW go haywire and it'd behave wrong even with new CFE. Then longer power-down reset it properly.