Do the rules surrounding international data transfers under the UK GDPR apply to transfers of such personal data back to the jurisdiction from which it originates?

We are instructed by a UK corporate with EU and US based subsidiaries in connection with certain data privacy arrangements. We are advised that personal data from customers in the US is processed in the UK and then transferred back to the US. Do the rules surrounding international data transfers under the UK GDPR apply to transfers of such personal data back to the jurisdiction from which it originates?

My reading of your practice note "UK GDPR: Overview" seems to suggest that it does, and I assume that (short of an exemption) BCR or SCC should be put in place in relation to such transfers. However, I would like to clarify my understanding because the effect would seem to be to require the US arm of the group to offer a GDPR equivalent standard of protection in relation to data which, but for its transfer to the UK, is or would be, subject to a potentially less onerous regime.