Since I've bought Zenfone 6 I could see that bluetooth device connecting takes more time than on some other phones. That was fine though. Recently I've bought BT5.0 True wireless earbuds (chinese, Blitzwolf BW-FYE7) and connecting with these often result in bluetooth crash and timeouts which isn't the case with SGS6 or Huawei Mate 10 Pro. More detailed description below:
-Blitzwolf BW-FYE7: bluetooth hangs while connecting in many cases(especially when enabling bluetooth while earphones are already on), rendering phone almost unusable for a while; entering settings will result in a crash, using Quick Setting may crash SystemUI, fiddleing a minute or two fixes the problem.
-Sony A400BT: establishing the connection can take up to few minutes(!) while using S6 that usually was around 30 seconds.
-JBL Flip 4: a minor issue, doesn't connect immediately (which again, is almost instant on some other phones) and it can take up to 1 minute.
Bluetooth can also hang in about 5% of other connections, provided I'll get sick of waiting and disable/reenable bluetooth. I can provide logs but I need to know which app to use and and when to start collecting them. I am quite certain this is just a software bug.
EDIT1: It seems bug reproduction scenario is a bit harder to make. Now my earphones connect flawlessly when I tried to hcidump bt. I will reply to myself when I'll be able to log it so there's no need to asap replies 🙂