Welcome to the Pi-hole community, Adam_Chilton. 
Your debug log looks normal.
There are a multitude of reasons why you would see WiFi connections reset or drop out, even if observed only for a limited number of clients. (click for more)
To give just one example:
A weather radar operating in your vicinity may periodically force your router to switch to another WiFi channel and not to reclaim it for a limited period.
On a dual channel router or AP, this may affect 5GHz channels and devices connected through it, while 2GHz continues to operate undisturbed.
But as you have reason to attribute this to Pi-hole, I am not going to dismiss your observation.
A probable way for Pi-hole to cause this would be that Pi-hole blocks DNS requests for some telemetry domains of your devices.
Different devices (or rather, their OSs) may employ different strategies to establish that they have internet connectivity, e.g. Android may try to connect to connectivitycheck.android.com (among others).
If Pi-hole blocks these domains, an OS will report that no Internet is available.
It then depends on OS and version how consecutive connection attempts are handled: Some may choose to actively suppress them (as they think they know that to be futile), and some may just let them pass, observe if an answer is received and update their connectivity status on success.
A potential mitigation would be to allow contact to those domains, but be aware that you are alerting additonal third parties of your internet usage that way.
In oder to find out which domains to unblock for your specific devices, you may want to have a closer look at How do I determine what domain an ad is coming from?
As a side note: Technically, Pi-hole cannot block access to the internet as such (click for details)
Pi-hole just filters DNS requests.
While DNS is fundamental to resolve domain names into IP addresses, solely those IP addresses are used for communication.
It may seem like you cannot reach anything on the internet, but if you would access web sites by their IP address rather than by domain name, you would still be able to access and retrieve data.
Granted, that would be quite cumbersome, so not a real option at all. 