When using Transparent mode, the Standard shader for Builtin Render Pipeline uses the following blend modes (https://docs.unity3d.com/Manual/SL-Blend.html):
ForwardBase pass: Blend One OneMinusSrcAlpha
ForwardAdd pass: Blend One One

These mean respectively:
outColor = 1 * src + (1 - src.a) * dst
outColor = 1 * src + 1 * dst
Where 'src' is the color produced by the shader, and 'dst' is the color present in the framebuffer already. These blend modes apply to the entire colors, which includes the alpha channel. The blend mode of the ForwardAdd pass is causing issues in this case, since the alpha value produced is being added onto the one existing in the framebuffer.

Unity supports specifying separate blending factors for both the rgb and alpha channels, which can be used to work around this. In particular, a blend mode of 'Blend One One, Zero One' may be desirable, since this means "blend the rgb values additively, but just take existing alpha value from the framebuffer and ignore the output of the add-pass". This change can be made to a copy of the Standard shader, for which the source code is readily available, or by creating a custom shader that fits the requirements.

We won't fix this issue as it is specific to the Builtin Render Pipeline, and can change the appearance of existing projects.