EU acted in good faith by not implementing a EU first clause in their vaccine production, meaning they were able to export large amounts and expected the agreements they had signed would be honored (mistake 1 - UK doesn't care about agreements). UK acted in bad faith by putting a UK first clause in their vaccine production, so very few exports and even vaccines produced in the EU by a UK company went to the UK first, despite existing agreements the company had with the EU. As far as "leniency on the Brexit deal" - it was a deal the UK agreed to and the UK got what they wanted so 'leniency' doesn't enter the picture.
The same applies to EU, they signed a deal with AZ knowing (or not it would seem initially) and they got what they signed for - best effort. Saying things like "We (the EU) acted in good-faith" is just PR that they tried to make themselves look like the victim, as back in January the EU when everything began, they didn't even seem to realize their agreement was best-effort. So saying it was agreed to in good-faith on "best-effort" is just nonsense as back then they were saying the agreement
guarantees quantity at certain dates, which turned out to be untrue. I don't believe for a second they made the conscience decision to not include a first clause as the first time the EU came out with "They acted in good-faith" is when it was determined the UK had a first clause in their agreement.
But putting that aside, where was this good faith when they EU initially said AZ was in breach and threatened them with legal action? Where was the good faith when they imposed the border block and ended up reversing it hours later, and still taking two weeks to sort of admit mistake? Where was the good-faith when they demanded the UK must export their vaccines to EU? Where was the good faith when the EU commission authorised blocking of vaccines to Australia? Every step of this debacle has been a hostile approach from the EU.
In simple terms, the UK signed a deal with EU, deal with it. The EU signed an agreement, deal with it! If EU wants something UK has, what have you got to offer in return?