From memory it's actually not that uncommon for the second half of April to be a crushing disappointment from a warmth perspective.
2016-17 both had some pretty potent cold snaps in their second halfs , in 2017 there were pretty damaging frosts in the last week which wreaked considerable havoc all over Europe in an otherwise exceptionally warm spring
Just over a quarter of Aprils have had a colder second half. 68 out of 252 to be precise. The years 2015, 16 and 17 all had colder second halves. 2016 had the biggest drop of recent years, 8.5°C for the first 15 days, 6.6°C for the second half.
Incidentally, by my calculations 2024 has been the warmest first half of April, at 11.2°C, just piping 2011's 11.1°C.