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That seems correct,
From openvpn(8):

If the algorithm parameter is empty, compression will be turned off, but the packet framing for compression will
still be enabled, allowing a different setting to be pushed later.

PR for 'disable' option:
https://github.com/pfsense/pfsense/pull/4161

Omitting the directive uses the OpenVPN default, which is what the option already says, and that was an intentional choice. The behavior beyond that depends on the OpenVPN version and what it does, which is NOT to disable. The very first option in the drop-down says to disable, and that's what it does.

This is not a bug but a support/discussion issue, which belongs on the forum and not here.

       --compress [algorithm]
          Enable a compression algorithm.

          The algorithm parameter may be "lzo", "lz4", or empty.  LZO  and
          LZ4 are different    compression algorithms,    with LZ4 generally of-
          fering the best performance with least CPU usage.     For backwards
          compatibility with OpenVPN versions before 2.4, use "lzo"    (which
          is identical to the older    option "--comp-lzo yes").

          If the algorithm parameter is empty, compression will be    turned
          off,  but     the  packet framing for compression will still    be en-
          abled, allowing a    different setting to be    pushed later.