Note: This article has the reference to the issue with the fix available:
Unable to add TXT DNS records longer than 255 characters in Plesk for Windows even if semicolons are used to split it record in blocks and bypass this issue.
DNS server does not serve requests to a domain with DKIM record longer than 255 characters.
Plesk for Windows: bug #PPPM-8657 which is planned to be fixed in the next Plesk version.
Plesk for Linux: bug #PPPM-3170 which was fixed in Plesk Onyx 17.5.3 Update 13
In Plesk for Windows, there is no workaround available at the moment.
In Plesk for Linux, upgrade Plesk to 17.5 or higher.