When typing on Bluetooth keyboards, letter sometimes infinitely repeats while not holding it down

For years, I've had no problem with using on a Microsoft Sculpt Mobile Keyboard as well as a Microsoft Bluetooth Mobile Keyboard 5000. I'm using a late 2011 MacBook Pro, OSX El Capitan v.10.11.3, (Bluetooth 4.0).

problem:
Starting about 3 weeks ago, sometimes I'd press a letter key, I'd let go of the pressed key, but OSX seemed to sense I was still holding the key down. This problem happened with both those keyboards. This problem never occurred while using the attached keyboard.

Once the letter started uncontrollably repeating, I switched to the MacBook's attached keyboard. After about 1 second of frantically pressing esc while also using the trackpad to place the cursor behind the repeating text, the last character typed stopped repeating. I could continue typing on the Bluetooth keyboard as if nothing had happened. The problem would not occur again for days.

ideas:

• This problem has coincided with my El Capitan upgrade.
• This problem definitely happens to me while doing Japanese character input. I'm not 100% sure that this has also happened to me while doing English input.
• El Capitan has added new features with regard to Japanese character input.
• Of course, the keymappings for the 2 MSFT Bluetooth keyboards are far apart from the unique / weird MacBook keyboard layout. They sure don't have fn, control, option, command, … and the command key is used for Japanese input.

So far, it has happened when I am typing in MSFT Word, MSFT Excel, a Chrome browser, and the NetBeans IDE. But not in any Apple apps.

But, it only happens infrequently. It is annoying, but not the end of the world. Maybe just buy an Apple keyboard.