@R C-R: That sounds a lot like a problem to me TOOOOO!
@AdamW: WHY the Hell?

I understand: Affinity will expire the beta, to sell the Beta-phase the «non-Beta». BUT WHY THE HELL in such a short a time?   #################### PLEASE, do change your practice, so at least the previous version will survive 2 following versions! 

Proposal for calculation:
exipreTime = releaseTime + 3x releaseIterations
If your plan is publish all 4 Weeks a new beta-release. The second-last beta will expire 2 x 4 weeks after the release of latest version publishing-time.
So when every four weeks:

Affinity Publisher Beta_1.7.0.162
Release Date: 05.11.2018
Expire Date: 05.02.2019

Affinity Publisher Beta_1.7.0.157 (previous)
Release Date: 05.10.2018
Expire Date: 05.01.2019

Affinity Publisher Beta_1.7.0.149 (second-last)
Release Date: 05.09.2018
Expire Date: 05.12.2018

That would give the possiblity for your BETA-Tester to use at least 1 or 2 (if previous also has a fundamental bug) version back.

AND for the business there is only 2-times of your release-time to make monetisation.

Official BUY-Release:
Affinity Publisher 1.8 (BUY-Release)
Release Date: 05.12.2018

Last BETA will expire 05.02.2019
####################

Im sure, your product will be (and is) such amazing, everty Beta-Tester will buy the price (if this is not extremely overpriced compared to «photo» and «designer»!
All other, not willing buyer will have only 2 month (2 releaseCycle) left to have an amazing Beta-App.


Cheers
Martin