When loading samples with greater than 16 bit precision, OpenMPT normalizes to volume automatically on load. This is done because OpenMPT internally only stores 16 bit samples and by normalizing them while loading, OpenMPT minimizes quantization noise that occurs when converting to lower resolution. To compensate for the increased volume due to normalization, OpenMPT adjusts the sample playback volume down accordingly. In your test file, sample 1 has global volume 13, while sample 2 has global volume 64. So, just increasing the volume of sample 1 back to 64 should give you the desired volume.
You can also disable the whole normalization-on-load behaviour by setting "Sample Editor.MayNormalizeSamplesOnLoad" to "0" in the advanced settings. Keep in mind though, that this might unnecessarily increases quantization noise when loading samples with greater than 16 bit precision that are not yet normalized.

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