Other items show extended info when sneaking. Drawers show no inventory info when sneaking.

Minecraft v1.12.1
All The Mods 3 v3.01
The One Probe v1.12-1.4.18
TOP Addons v1.12-0.15.1

Thank you, I hadn't realized that config was server-side and was more nuanced than on/off.

The "Not needed" and "Extended" settings appear to operate the same, both requiring the probe to see extended information. ("Needed" behaves as expected.)

Edit: /topneed seems to be tied, in part, to what the config file is set to. If the config is set to 3 (Extended), /topneed can be set to "Not needed" but the probe is still required for extended info. It seems as though the config file sets an upper limit to how permissive this is but /topneed doesn't reflect this.

/topneed setting also seems to be stored separately from the config file. Changing the setting via /topneed isn't reflected in the config.

Is /topneed client-side only, being "capped" by the server config?

Normal behavior is that not needed shows everything, with and without probe equipped. If some kind of client/server mishap is happening that must be on The One Probe's side, but I'll test this case out more with a server.

If your server/single player has the needsProbe setting on Extended the extended drawer information only shows up when you have a probe, this currently also makes the default info disappear.

Please try sneak-looking at a drawer again while holding a probe or with the setting on Not needed.

You can change the setting with the /topneed command or by right-clicking the note you get at the start.

As near as I can tell, /topneed only effects the client (or single-player). The server config, of course, affects the server. The server config limits the client setting. This seems all normal unless you happen to be op and expect command line stuff to effect the server via the client. In retrospect, the fact that /topneed pops up a gui, rather than directly controlling the setting, is an indicator that it may be only effecting the client.

Given the topic for this issue and the fix you provided, I'm closing this issue. I think /topneed works as intended, but can be slightly confusing if you make certain assumptions due to circumstances.