On my Axe III and FC-12, If a hold function is assigned, it times on the release of the switch when using Last Two mode for the tap tempo. I'm on 20.04 and 1.12 FW's respectively, but it's done it this way for quite a few FW versions before.
To test, I set the hold timeout to 2 seconds to avoid triggering the hold function (my Hold Function Mode is set to Timeout). I can tap repeatedly at roughly 120 BMP and on the last tap, hold the switch for about a second or so then release and the tempo I get is around 40 to 60 BPM and NOT around 120. If it was timing by the down it should give me roughly 120 BPM as that's rate at which the presses are happening.
If I remove the hold function, it does indeed time on the down and ignores the release as expected.
I can confirm that the tap tempo is affected by an underlying hold function.
Gear
AXE FX3 MkII, firmware 20.04
FC-12 MkII, firmware 1.12
Settings AXE FX3
Hold function timeout: 2 sec
Hold function mode: timeout
Tap tempo: average
Settings FC-12
Tap: Tap tempo
Hold: Delay 2 (not in preset)
Procedure
I set a metronome to 110 BPM and consider clicks as 1/8th notes.
a) I press the switch on the 1rst, 3rd, 5th and 7th click for two bars, while I lift the switch on the 2nd, 4th, 6th and 8th click.
b) Then for two bars, I move the lift forward half a click (on the 1/16 note between the 1 and 2, 3 and 4, 5 and 6, 7 and 8).
a) as a above
b) as b above
I do this without interuptions. This gives an unsteady BPM at around 55 BPM. I gets most unstable when I switch between a and b and back again.
If I disable the hold function, I get a steady BPM at around 55 BPM when following the same procedure.
Conclusion
When no hold function is programmed, the tap tempo follows the press. When a hold function is programmed, the tap tempo appears to follow the lift.