Unable to change output sink on some apps

[himself@xxxxxxxxxxxxxx (John Allie)]

Hello,
I have a pair of Bluetooth headphones paired to the computer. Some apps are
capable of playing through them if selected in pavucontrol. Others, though,
won't let me select the headphones. In fact, I can't change the output on
those apps at all. I can click the button and see a list, but if I select a
different option it just goes straight back to HD-Audio Generic. Other apps
(such as Spotify) allow me to switch from one device to another without
issue.

I've also tried to change the sink via the command line using pactl but for
some reason it returns "Invalid Argument."

Here's the sink-input data from pactl:

Sink Input #8
        Driver: protocol-native.c
        Owner Module: 11
        Client: 24
        Sink: 0
        Sample Specification: float32le 2ch 44100Hz
        Channel Map: front-left,front-right
        Format: pcm, format.sample_format = "\"float32le\""
format.channels = "2"  format.channel_map = "\"front-left,front-right\""
        Corked: no
        Mute: no
        Volume: front-left: 65536 / 100% / 0.00 dB,   front-right: 65536 /
100% / 0.00 dB
                balance 0.00
        Buffer Latency: 54807 usec
        Sink Latency: 23177 usec
        Resample method: copy
        Properties:
                media.role = hex:
                phonon.streamid = hex:
                media.name = "Playback Stream"
                application.name = "bioshock.i386"
                native-protocol.peer = "UNIX socket client"
                native-protocol.version = "26"
                application.process.id = "10390"
                application.process.user = "john"
                application.process.host = "strangelove"
                application.process.binary = "bioshock.i386"
                application.language = "C"
                window.x11.display = ":0"
                application.process.machine_id = [redacted]
                application.process.session_id = "2"
                module-stream-restore.id =
"sink-input-by-application-name:bioshock.i386"

I'm running Linux Mint 18.3 "Sylvia", KDE Plasma 5.8.9, KDE framework
5.36.0, and pulseaudio 8.3. The stubborn app that won't switch devices is
BioShock Infinite, from Steam. I also tested The Talos Principle (also from
Steam), 64-bit version, and it wouldn't allow me to change the output
either. Could Steam be part of the problem?

I previously posted this question on StackExchange but no one there seems
to know the answer. You can view that thread here (includes screenshot):
https://unix.stackexchange.com/questions/452907/pavucontrol-wont-change-output-on-some-apps

I also found this description of a very similar problem, although it was
never solved either:
https://unix.stackexchange.com/questions/272499/pulseaudio-cannot-change-the-sink-for-one-specific-program

Thanks for any suggestions!
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