New felt flat roof - wrinkles?

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Hi everyone.

I have a flat roof shared with the neighbour (semi detached). The whole roof was covered in felt at the time of installation decades ago and I wanted our side replaced (neighbour not interested in spending money replacing hers). The roofers (who did a great job on our tiled roof and have an excellent reputation) did our side over 2 days. They primed the wood, laid underlay, tar layer stuff and then felt. They used the blowtorch etc. and also installed flashing. All looked fantastic. However, 2 days later I had a look and there are slight wrinkles across the roof! To the touch, it's solid - no bubbles. Some times of the day its barely noticeable but other times (early morning) or after a hose down, it's very noticeable. The roofers are speaking with the manufacturer as they have never seen this happen before and are using premium materials (I checked). There was no rain when it was installed, but it was hot - around 26C. I've hosed it down and so far no leaks. But I cannot find anything online that would indicate the cause and they're stumped too.

Any ideas? I've seen the roofs they've done (hundreds) for other customers and they're great. The work is very neat as well. They're not fly-by-night cowboys, but a professional outfit. Is the issue the weather at the time, or the materials or the installation?

Photos 1-3 are early morning and 4-5 are late afternoon after a brief hosing down to test water resistance.

Thanks for any insights!
 

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Any ideas?
I can only think that if the felt was laid when the roof/building was hot, subsequent cooling and contraction of the building would generate wrinkles if the felt didn't contract by the same amount.
 
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That would make sense - the question then is, is this likely to pose a problem over the next 10-15 years or can it be ignored? The aerial photos make the wrinkles look much bigger than they are. When I'm kneeling on the roof, I can barely see them.
 
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Actually - could it be caused by the massive overlap of each row? When wet, you can see like they overlap each other in the middle of each sheet, instead of at the edges..
 

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