DMARC rejection after Exchange upgrade

I'm having problems with inbound emails getting bounced as Undeliverable due to DMARC rejection.

For many years I've had my email come through Fasthosts / Livemail to my own domain (qts.org.uk) with catch-all forwarding set to forward everything to my GMail account. Just recently Fasthosts have upgraded their servers to Exchange and I've started getting DMARC rejections from GMail which start

Diagnostic information for administrators:

Generating server: exchange2019.livemail.co.uk

Total retry attempts: 1

(my gmail email address)

t1-hex-xprelay.gem.livemail.co.uk

Remote Server returned '550 5.7.26 Message rejected by DMARC policy by gmail.com. Please use your own email address as the sender, instead of (sender's email address). [MSG0009]'

Which bounce from Fasthosts / Livemail back to my GMail address.

My own domain has SPF, DMARC, and DKIM configured

I've done a little digging and it appears to only affect senders from originating domains with DMARC set to reject.

So either GMail has coincidentally become much more strict (possible) or Fasthosts are somehow failing to forward emails fully transparently.

I have spoken to Fasthosts and logged the issue with them and was not impressed so I hope the experts here can offer a solution I can forward to them.