Behaviour change between ITK 5.3 and 5.4 with DICOM spacing

Further to some discussions in https://github.com/InsightSoftwareConsortium/ITK/pull/4647 and https://github.com/InsightSoftwareConsortium/ITK/pull/4521, we still have a situation that isn't quite clear to us, and since those are merged already, we thought we'd create a new ticket.

Basically, there is a change in behaviour between ITK 5.3 and 5.4 that we are trying to understand if it's a bug or deliberate & correct.

If we query a DICOM file for its image spacing information, we get different results depending on:
- using `itk::ImageSeriesReader` vs using `itk::GDCMImageIO`
- having 1 DICOM file in the folder vs having multiple

A small C++ test case is attached: [inconsistency_in_spacing.zip](https://github.com/user-attachments/files/16473741/inconsistency_in_spacing.zip)

It uses a DICOM file named `D_CLUNIE_CT1_J2KR1.dcm`, from the GDCM test suite I think, but from David Clunie's collection ultimately I suppose.

In ITK 5.3:

Number of files | ImageSeriesReader | GDCMImageIO
---|---|---
Single | [0.661468, 0.661468, 1] | [0.661468, 0.661468, 1]
Multiple | [0.661468, 0.661468, 1] | [0.661468, 0.661468, 1]

Notice everything agrees: the z spacing is 1.

In ITK 5.4 / master:

Number of files | ImageSeriesReader | GDCMImageIO
---|---|---
Single | [0.661468, 0.661468, 5] | [0.661468, 0.661468, 5]
Multiple | [0.661468, 0.661468, 1] | [0.661468, 0.661468, 5]

Notice now everything does not agree.

The change from 1 to 5 is presumably deliberate from https://github.com/InsightSoftwareConsortium/ITK/pull/4521.

But then shouldn't that lone 1 (in the bottom left) also be 5?

Thanks.