it's probably because it is used as part of something else, Virtual Wire Pair, Aggregate, SD-WAN..
Aren't there any collapsed parts/interface listings in GUI / Network / Interfaces ?
How about to use FortiOS CLI to find it, via config listing:
show full | grep -f port10
Or check for references (just example output):
FGT3HD-2 (root) # diagnose sys cmdb refcnt show system.interface.name port6
entry used by child table member:interface-name 'port6' of table system.virtual-wire-pair:name 'test'
Tom xSilver, planet Earth, over and out!