Sourdough cracking along top while baking

Hello,

I'm new here, but i've been baking sourdough bread for about three years (so my starter is/was pretty healthy) in a completely intuitive/ unprofessional manner. Any advice would be MUCH appreciated!

Two things have changed - i have an amazing new loaf tin with a lid (bought from No-Knead Baking), and also i have been away for six weeks.

I have successfully baked using the new tin a few times(while i was away, using a different starter, one i made from scratch in situ): in fact, the bread was my best ever.

My first loaf back home all was going well and nothing looked any different after proofing (overnight) but in the oven it developed a huge crack running the length of the top.

The second loaf, five days later, the dough didn't rise as much during the overnight (a slightly colder night) proof and the crack was even bigger (see pic).

I fed up my starter over three days before commencing and i'm sure it's alive and well as it is bubbling away. But maybe it's not yet up to full health/strength?

I use a mix of einkorn and organic white spelt (about 40/60); nothing has changed there. Possibly the dough was wetter than previously but if so, not noticeably. 

I didn't make any slashes on the surface, but i never have in the past. I meant to this time after reading some troubleshooting, but forgot until the morning so i half-heartedly slashed the top about 45 mins before baking.

Thanks for reading this far!!