Specifically, any permutation using "@All" not on its own such as:
@All and Form = 'Person'
results in a message of "Query is not understandable - syntax error - MUST have at least one operator", with "Call hint: OSLocalAlloc, Core call #0". This is the same error that happens if you use an @function that doesn't exist in DQL, like "@foo", or a bareword as the whole query, like "foo".
Would like to request that "@All" in DQL should work similarly to how it does in formula-based queries, where it's essentially a no-op that matches all documents but is nonetheless legal to combine with other syntax.
While there's no real case where a human would manually compose a query combining "@All" with other criteria, it is very plausible for a query post-processor to do so. For example, a program might accept user input (either of DQL directly or abstract terms) to make a query and then combine it with "and Form = 'Person'" to restrict the query to only documents for a given form. Keep's current development state.
The immediate impact is that any such processor must now compare the incoming query to see if it case-and-whitespace-insensitive matches "@All" and operate differently on it, or to use a baseline matcher like "not (Form = '')" if such a thing can apply. This is not a showstopper, but it's inconvenient and unexpected.