Why is "fhepfcelehfcepfffacacacacacacabn" a top DNS query from my devices?

That domain is an encoded form of the string "WORKGROUP". It is using a variant of hex encoding that uses the letters A-P, instead of the numbers 0-9 followed by A-F.

$ echo fhepfcelehfcepfffacacacacacacabn |
  tr a-p 0-9a-f |
  xxd -r -p |

00000000: 574f 524b 4752 4f55 5020 2020 2020 201d  WORKGROUP      .

This appears to be a NetBIOS name, which is why it's padded with spaces to 15 ASCII characters, and then followed by a different character at the end as a suffix. The hex encoding is described in the NetBIOS-over-TCP/UDP Concepts RFC, called "first level encoding". Also, NetBIOS uses DNS record type ID 32 for its "name service" packets; that ID was later allocated to NIMLOC (ref), which explains that part of the log. However I'm not sure exactly what software on your machine is making this DNS query; if you're using Windows, it seems likely to be something at the OS level.

I don't believe the answer from NextDNS support is correct here about the source of this particular query—it is probably not coming from Chrome.

Update: This answer by Miles is a better insight, the explanation given by NextDNS support seems wrong.

I contacted NextDNS support asking for more details and they said this is Google Chrome testing internet connectivity.

Knowing where to look, I found numerous references for the same behavior: This article from 2012 discusses the mechanics with similar jumbled domains, albeit shorter strings. This Chromium ticket from 2010 reports a bug in the related behavior and also shows similar garbage domains.

In conclusion, this seems safe, albeit unexpected.