Not neccesarily… The diaphragm on The Ones vents out through that cap, and it’s possible for old fluid or even random water to gather in it, which gets pushed out when you back up the calipers. So it may not be leaking at all (or, perhaps, not by a worrisome amount, I suspect the bellows is slightly permeable) OTOH yes the diaphragm does sometimes puncture especially on the first year model- they changed it slightly to make it smaller, which I think prevents it contacting the inside of the m/c.
The only part that can have failed to cause a leak there is the diaphragm though and it seems to be £2.50, easily replaced too though you might need circlip/pin pliars. I have no idea why it caused bleed issues when mine went, but it did, mine is not to reason why…