Why is a Linux host interpretting the zero key as enter?

I am connecting to a Debian virtual machine using PuTTY, and every time I press the 0 (zero) key, the command line behaves as if I've pressed enter. Why might this be and how can I change it?

Perhaps also relevant: I'm connecting from Windows 10, using PuTTYtray, and have installed a custom keymap created using Microsoft Keyboard Layout Generator 1.4 (which leaves 0 alone but changes its shifted behaviour). The Debian machine is a DigitalOcean droplet. I've been using it for about a year and have never noticed anything like this.

Things I've tried:

• Pressing 0 in other applications -- normal behaviour
• Pressing 0 in other PuTTY sessions on other hosts -- normal behaviour
• Trying another shell (bash instead of zsh) -- 0 still acts as enter
• Disconnecting and reconnecting -- same behaviour
• Keypad 0 -- also acts as enter
• Shift and 0 -- works as expected (> on my keyboard)
• Connecting to the problem host via another (Ubuntu) host -- 0 acts normally
• Connecting to another host via the problem host -- even weirder! -- 0 acts as if I had pressed 0 THEN enter
• GNU screen -- 0 still acts as enter

Any suggestions?