I guess getting advice from Costco is best but could the battery of the PartnerMic be crapping out? If you have a USB digital multimeter, which come pretty cheap on Amazon, you could fully discharge, then fully recharge the PartnerMic and with the multimeter measure the mAh charge consumed on charging fully. Because USB charging is done at 5.0 volts, you just have to remember to multiply the mAh figure by 5.0/3.7 (or whatever the PartnerMic says it’s mAh capacity is rated at for average discharge voltage). If Phonak doesn’t provide any of this in the PartnerMic literature, you can probably find out by getting the FCC ID (required to be on the device and in the literature) and search the web on that ID for the FCC teardown analysis. Every time I’ve done this, e.g., for ReSound Multi Mic and Phone Clip+, I’ve found pictures that show the battery capacity and the average discharge voltage at which the device is rated.
Because my grandkids have one, I recently bought myself a Nintendo Switch - it’s a pretty amazing device even though it’s only a 1080p display. But I’ve had problems with letting the Joy-Con controllers sit around and self-discharge. A bad battery makes a controller behave really funky. Same thing with a Samsung watch - although in that instance I think the Samsung Health app du jour was causing excessive battery drain and make the watch behave crazy 'til I replaced the battery. But perhaps even though you can charge up the device, it might not have the capacity and the “youthfulness of battery” to delivery the current demanded at the voltage required. Especially if you bought a used device that’s seen a bunch of wear already such as being kept at 100% charge or kept in hot conditions, etc.