I know this is an older post, however i to am having the same issue. 620B on 5.0.10 with webfilter and explicit proxy enabled.
Did you find a solution to this?
Fortigates - FWF60D/FWF90D/FGT200D/FGT620B/FGT1000C FortiAPs -
FP221C/FP320C
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