Your system information

System:
  Host: mx Kernel: 6.13.6 arch: x86_64 bits: 64 compiler: gcc v: 12.2.0
    Desktop: Xfce v: 4.20.0 tk: Gtk v: 3.24.38 wm: xfwm dm: startx
    Distro: MX-23.5_x64 Libretto January 21 2024 base: Debian GNU/Linux 12
    (bookworm)
CPU:
  Info: 6-core model: AMD Ryzen 5 5600X bits: 64 type: MT MCP arch: Zen 3+
    rev: 0 cache: L1: 384 KiB L2: 3 MiB L3: 32 MiB
  Speed (MHz): avg: 3343 high: 4303 min/max: 550/5405 boost: disabled cores:
    1: 4283 2: 2016 3: 2016 4: 4269 5: 4303 6: 4298 7: 4298 8: 4299 9: 2016
    10: 2016 11: 2016 12: 4294 bogomips: 103197
  Flags: avx avx2 ht lm nx pae sse sse2 sse3 sse4_1 sse4_2 sse4a ssse3 svm
Graphics:
  Device-1: Intel driver: xe v: kernel pcie: speed: 2.5 GT/s lanes: 1 ports:
    active: HDMI-A-3 off: HDMI-A-1 empty: DP-1, DP-2, DP-3, HDMI-A-2, HDMI-A-4
    bus-ID: 0d:00.0 chip-ID: 8086:e20b
  Display: server: X.Org v: 1.21.1.7 compositor: xfwm v: 4.20.0 driver: X:
    loaded: modesetting unloaded: fbdev,vesa dri: iris gpu: xe display-ID: :0.0
    screens: 1
  Screen-1: 0 s-res: 1920x1080 s-dpi: 100
  Monitor-1: HDMI-A-1 mapped: HDMI-1 note: disabled model: DP1080P60
    res: N/A dpi: 94 diag: 595mm (23.4")
  Monitor-2: HDMI-A-3 mapped: HDMI-3 pos: primary model: Samsung
    res: 1920x1080 dpi: 305 diag: 184mm (7.2")
  API: OpenGL v: 4.6 Mesa 25.1.0-devel (git-2e3b23539e) renderer: Mesa
    Intel Graphics (BMG G21) direct-render: Yes

The issue:

If you open the steam client at 1080p then change to 1440p and change to big picture mode, then change back to 1080p and close big picture the UI scaling will be off. Everything will either be really huge and not fit on screen, or it will be super tiny and not usable. This requires to restart the Steam client to rectify.

Bigpicture mode seems to handle the resolution change fine, but when you exit big picture is when the scaling is affected.