@Pcoombe reported this and I have confirmed myself locally submitting a donation in GBP using 'variant=upsell'
It looks like something is misbehaving in the {{ (currency ~ " " ~ amount) | l10n_currency(locale) }} call within the recurring_notification.en.html template rendering within https://gerrit.wikimedia.org/g/wikimedia/fundraising/crm/+/refs/changes/10/539210/7/sites/all/modules/thank_you/templates/html/recurring_notification.en.html#6
I also remember this patch went live which *shouldn't* be related but it touches something similar so it might be worth looking at if nothing obvious stands out https://gerrit.wikimedia.org/r/#/c/wikimedia/fundraising/crm/+/540232/