I neglected to include a relevant fact in my original post. My config file for building 4.19.50-1 makes some changes to the stock config:
$this->bbcode_second_pass_code('', '$ diff config config.original
3c3
< # Linux/arm 4.19.50-1 Kernel Configuration
---
> # Linux/arm 4.19.49-1 Kernel Configuration
2836c2836
< CONFIG_I2C_BCM2835=y
---
> CONFIG_I2C_BCM2835=m
4945,4946c4945
< CONFIG_RTC_HCTOSYS=y
< CONFIG_RTC_HCTOSYS_DEVICE="rtc0"
---
> # CONFIG_RTC_HCTOSYS is not set
4958c4957
< # CONFIG_RTC_INTF_DEV_UIE_EMUL is not set
---
> CONFIG_RTC_INTF_DEV_UIE_EMUL=y
4966c4965
< CONFIG_RTC_DRV_DS1307=y
---
> CONFIG_RTC_DRV_DS1307=m
')I'm speculating that the change of CONFIG_I2C_BCM2835 from "m" to "y" (in order to build the driver in) is upsetting the expected order in which things are called.
Note that 4.19.49-1 (and previous) built with the same edited config file works properly.
UPDATE:
It has been confirmed that the problem appears when the I2C-BCM2835 driver is built-in to the kernel. Further progress may be followed at:
https://www.raspberrypi.org/forums/viewtopic.php?f=44&t=242856UPDATE:
The problem has been resolved with the release of kernel 4.19.57-1.