SUMMARY
When switching brush in other tools, Brush Opacity will be exchange in these situation:

1) The Tool has No Opacity Available:
Brush A and B exchange Opacity. Move Tool is the exception which only copy Opacity A to Brush B

2) The Tool With Its Own Opacity:
Brush A give Opacity to B, B to the Tool, Tool to Brush A

Tested with 4.4.2, 5.0-beta1, 5.0-beta2, 5.1.0-prealpha-6e8aab8fd8 all have this bug.
Duplication of Bug 438346 but with some more detail.

STEPS TO REPRODUCE
1. Open Brush History Docker, which can pick brushes when is not using Freehand Brush Tool
2. Select Freehand Brush Tool, and choose 2 brushes Opacity is not 100%
3. Select Brush A, and switch to tools other than Freehand Brush
3. Switch to Brush B when in that tool
4. Switch back to Freehand Brush Tool

OBSERVED RESULT

1) The Tool has No Opacity Available:
Brush A and B exchange Opacity. Move Tool is the exception which only copy Opacity A to Brush B

2) The Tool With Its Own Opacity:
Brush A give Opacity to B, B to the Tool, Tool to A,


EXPECTED RESULT
Each Brush and Tool should hold its own Opacity

SOFTWARE/OS VERSIONS
Windows 10 20H2
Opensuse 15.3 / Plasma 5.18.3
Android 11

ADDITIONAL INFORMATION

Detail test here:

Brush A: Opacity 60% (Pencil-1 Hard)
Brush B: Opacity 15% (Pencil-3 Large 4B)

Test Method: Freehand Brush Tool-> Brush A-> Other Tool ->Switch to Brush B-> Switch Back to Freehand Brush Tool

Layout:
'ToolName'
*(Tool Opacity Availability):'Opacity Property Move'

--------------------
Select Shapes Tool
*(No Opacity Available):A and B exchange

SVG Text Tool
*(No Opacity Available):A and B exchange

Edit Shapes Tool
*(No Opacity Available):A and B exchange

Calligraphy
*(No Opacity Available):A and B exchange

Freehand Brush Tool------

Line Tool
*(With Its Own Opacity):A to B, B to Tool, Tool to A

Rectangle Tool
*(With Its Own Opacity):A to B, B to Tool, Tool to A

Ellipse Tool
*(With Its Own Opacity):A to B, B to Tool, Tool to A

Polygon Tool
*(With Its Own Opacity):A to B, B to Tool, Tool to A

Polyline Tool
*(With Its Own Opacity):A to B, B to Tool, Tool to A

Bezier Curve
*(With Its Own Opacity):A to B, B to Tool, Tool to A

Freehand Path Tool
*(With Its Own Opacity):A to B, B to Tool, Tool to A

Dynamic Brush Tool
*(With Its Own Opacity):A to B, B to Tool, Tool to A

Multibrush Tool
*(With Its Own Opacity):A to B, B to Tool, Tool to A

Transform Tool
*(No Opacity Available):A and B exchange

Move Tool
*(No Opacity Available):A to B

Crop Tool
*(No Opacity Available):A and B exchange

Gradient Tool
*(With Its Own Opacity):A to B, B to Tool, Tool to A

Color Sampler Tool/Color Picker
*(No Opacity Available):A and B exchange

Colorize Mask Editing Tool
*(With Its Own Opacity):A to B, B to Tool, Tool to A

Smart Patch Tool
*(With Its Own Opacity):A to B, B to Tool, Tool to A

Fill Tool
*(With Its Own Opacity):A to B, B to Tool, Tool to A

Assistant Tool
*(No Opacity Available):A and B exchange

Measure Tool
*(No Opacity Available):A and B exchange

Reference Image Tool
*(No Opacity Available):A and B exchange

Rectangular Selection Tool
*(With Its Own Opacity):A to B, B to Tool, Tool to A

Elliptical Selection Tool
*(With Its Own Opacity):A to B, B to Tool, Tool to A

Polygonal Selection Tool
*(With Its Own Opacity):A to B, B to Tool, Tool to A

Freehand Selection Tool
*(No Opacity Available):A and B exchange

Contiguous Selection Tool
*(No Opacity Available):A and B exchange

Similar Color Selection Tool
*(No Opacity Available):A and B exchange

Bazier Curve Selection Tool
*(No Opacity Available):A and B exchange

Megnetic Selection Tool
*(No Opacity Available):A and B exchange

Zoom
*(No Opacity Available):A and B exchange

Pan Tool
*(No Opacity Available):A and B exchange