how does ssh cause this loop to preterminate?

[gregrwm]

Configuration Information [Automatically generated, do not change]:
Machine: x86_64
OS: linux-gnu
Compiler: gcc
Compilation CFLAGS: -g -O2 -Wno-parentheses -Wno-format-security
uname output: Linux ginger 4.18.0-240.15.1.el8_3.x86_64 #1 SMP Mon Mar 1
17:16:16 UTC 2021 x86_64 x86_64 x86_64 GNU/Linux
Machine Type: x86_64-pc-linux-gnu

Bash Version: 5.1
Patch Level: 0
Release Status: release

Description:
        [Detailed description of the problem, suggestion, or complaint.]

Oddly, somehow, invoking ssh in a loop causes the loop to preterminate.
Why?

Repeat-By:
        [Describe the sequence of events that causes the problem
        to occur.]

> $ #without ssh, as expected:
> $ (echo a;echo b;echo c)|while read a;do ((hostname;uname -r);echo
rc=$?)|while read x;do echo a=$a x=$x;done;done
> a=a x=ginger
> a=a x=4.18.0-240.15.1.el8_3.x86_64
> a=a x=rc=0
> a=b x=ginger
> a=b x=4.18.0-240.15.1.el8_3.x86_64
> a=b x=rc=0
> a=c x=ginger
> a=c x=4.18.0-240.15.1.el8_3.x86_64
> a=c x=rc=0
> $
> $ #with ssh, why does the loop preterminate?
> $ (echo a;echo b;echo c)|while read a;do (ssh 192.168.96.150
"(hostname;uname -r)";echo rc=$?)|while read x;do echo a=$a x=$x;done;done
> a=a x=g150
> a=a x=5.4.0-66-generic
> a=a x=rc=0
> $