Bug report

Bug summary

#19153 introduced some changes that had (I think) unintended consequences when passing custom kwargs to projections

Code for reproduction

Prior to #19153, the following code returned False, that is, two different axes were created:

from astropy.wcs import WCS
import matplotlib.pyplot as plt

wcs1 = WCS(naxis=3)
wcs1.wcs.ctype = ['x', 'y', 'z']

ax1 = plt.subplot(1, 1, 1, projection=wcs1, slices=('x', 'y', 1))

wcs2 = WCS(naxis=3)
wcs2.wcs.ctype = ['a', 'b', 'c']

ax2 = plt.subplot(1, 1, 1, projection=wcs2, slices=('x', 2, 'y'))

print(ax1 is ax2)

No deprecation warning or future warning was raised in this case.

Following #19153, this silently ignores the projection and slices in the second call and the print statement returns True.

I don't think #19153 was intended to start making plt.subplot return the same axes in some cases, rather I thought the goal of that PR was to remove re-use of axes in certain circumstances, so I think this is a regression/bug

Actual outcome

ax1 and ax2 are the same

Expected outcome

ax1 and ax2 should be different