Given the sheer volume of Cloudflare hosted DNS that operates without issue, you can safely ignore those erroneous results from MXToolbox. You can easily confirmt hat your serial is valid by comparing it with the requirements.
7. The DNS SOA serial number
The serial number in the DNS SOA Resource Record is a Serial Number as defined above, with SERIAL_BITS being 32. That is, the serial number is a non negative integer with values taken from the range [0 … 4294967295]. That is, a 32 bit unsigned integer.
MXToolbox also chooses to display a warning if the EXPIRE value isn’t in a range recommended by RFC 1912. A recommendation is not a requirement, so that too can be safely ignored.