As we discussed over on github (thanks for your help investigating this), it was determined that this is a design flaw in the ATtiny1634 chip; it is listed at the very bottom of the datasheet in the errata section (what a very conspicuous location for it) - PB3 (Arduino pin 14) does not function reliably as an input when the watchdog timer is not running (for both analog and digital reads), and is internally (weakly) pulled down. A note describing this will be added to the pinout diagram on the product page and core documentation.

It works fine as an output. If the watchdog timer is enabled, it works fine for both digital and analog reads.

Thanks!

That's a good suggestion - I hadn't thought of writing up a guide to migrating to ATTinyCore from other cores, and to ATTiny841 from ATTiny84.

The main differences are:
ATTiny841 has two hardware serial ports, named Serial and Serial1; they can be used like the serial ports on any other AVR through ATTinyCore. The ATTiny84 has none (though if using my core there is a builtin software serial implementation that uses the ACO vector, so it can be used without interfering with use of PCINTs for other purposes like the SoftwareSerial library would - see the ATTinyCore documentation for details - probably not relevant to you).

ATTiny841 has an additional 16-bit timer, timer2, which is identical to timer1. The PWM pin assignments are also more flexible - you can move the PWM outputs around between pins; this can allow some behavior that could never be achieved on most AVRs (unfortunately the coolest thing I did with this is covered by an NDA, so I can't go into further detail)

The ATTiny841 has a hardware SPI, and hardware I2C slave interface, while the ATTiny84 has a USI.
If using the Arduino IDE with ATTinyCore, you can use unmodified libraries designed for SPI or I2C devices, and the version of Wire and SPI libraries included with ATTinyCore for the 84 or 841 (these versions of the library automatically use the correct implementation of I2C or SPI for the hardware, and provide a compatible interface, so no changes to the library are needed). With the Arduino IDE and other cores that support the ATTiny84, you needed to use a different library for SPI/I2C that used the USI, and modify the library for the SPI or I2C device to make use of that library instead of the standard Wire or SPI library. If you are not using the Arduino IDE, you need to handle this yourself (the most notable hardship is that I2C master requires a software implementation of I2C - ATTinyCore handles this for you in Arduino-land, but if using Atmel Studio or other tools, you'd have to do this yourself)

On the ATTiny84, the internal oscillator is pretty close to 8MHz across the whole operating voltage range (ie, close enough for serial to work), but strongly temperature dependent (particularly on the low end). On the ATTiny841, the internal oscillator is very close to 8MHz across the whole temperature range below 4v, but the speed increases rapidly as voltage increases above that - such that above ~4v, it's not close enough for serial to work if you don't account for that (typically by adjusting OSCCAL). If using a crystal, which I recommend for any application where serial functionality is used (on any processor, not just this one), this is not relevant.

I've added an issue to ATTinyCore on github regarding this; you can follow this for updates on the progress towards more formal documentation. https://github.com/SpenceKonde/ATTinyCore/issues/285

-Spence