I also use PreP (Truvada) however don't currently hold a Class 3 any longer but when I did the restrictions were similar, however in another region. The CAA uses the HIV protocol as a base-line as the med can be used for those starting ARVs if they have the Virus and then obviously for PEP and PreP. The 4 week restriction of grounded is normal due to some having debilitating side-effects initially (Cramping most common). I under go blood monitoring on 3 monthly intervals (discussing moving to 6 with my Doctor who is also an AME).
If the person is already on PreP at application then it depends on the CAA/AME if the person's GP has the records side-effects and of the blood monitoring at the required intervals (3 Months) then they would apply to the CAA medical board for an unrestricted certificate, if the Blood monitoring is not available and the person's GP did not monitor side effects or the initial blood test wasn't done, then the person will usually get the 4 week restriction whilst the monitoring can be done.
To explain from the CAA expectation - the AVMED panels look at the chances of incapacitation occurring and for a medical to be issued as unrestricted the likelihood of ATC incapacitation should be around 1% of the time the ATC is on position (a rather arbitrary and difficult number to prove but in theory it is the discussion that is held) if the medication has a likelihood to induce side-effects that could cause you to become incapacitated (this does not mean passing out and not being able to control but more needing to leave your position for a period whilst on duty- so this includes the likelihood of diarrhoea or stomach cramps causing you to have to leave your position even momentarily would count as such incapacitation case). Although these are normally manageable in most people the AME would have to monitor your reaction to the side effects and hence the 4 week grounding period. As with any medication (including Beta Blockers or any other meds that could cause major side effects) the Medical is usually temporary suspended while the AME monitors the applicants reaction to the med and the side effects.
As more information is gathered on these, A protocol may be developed by the AVMED panels or ICAO and this would then be used as a norm. Truvada is still relatively new (even your AME didn't know what it was) so the data to develop a protocol will be coming from persons like you and I who are doing the regular monitoring etc. In a few years (unfortunately yes years the process to get the data for a new protocol is rather lengthy) if it is determined that the majority of Medical holders did not experience debilitating side-effects in the 1st 4 weeks and that there is little difference in 3 month or 4 month or 6 month blood results or the ability for such intervals to still pick up major problems then a protocol may be issued that lessons the restrictions and also precludes these cases going to medical panel all the time, only if an applicant displays symptoms outside of the protocol -additional side-effects or blood results at a certain level etc). For this reason I would suggest you keep on the programme so that more data is collected at your CAA-AVMED to allow for future sharing of data, however if this interferes too much with your license then stopping would be easier.