Your first initial email looks correct, but after replying from the Mail app on iPhone part of the signature overlays the previous email.
Overlapping is a well-known issue for the Apple community already for ~3 years; please check this article for more information. It has not been officially fixed yet, but there are some workarounds, which you might find below.
1) Ensure that the images used within your signature design are created at the required size for the signature template.
Apple Mail Client is trying to use the Width and Height values set on the image(s) in the signature template while trying to revert the image(s) to the Native image size. Sometimes this can occur when sending an email from a mobile device and viewed on an Apple Mac and or Tablet.
We recommend uploading a logo/personal image as a .png file with a high resolution and high DPI (300). For example, if your logo appears as a 150x150 image in the signature, then a 450x450 image must be uploaded; what's more important - the image's resolution should be divisible by 2, 5, and 10, for example, 200x200 or any other round numbers, and the banner-image keeping the ratio 1:5 or 1:6. It is strongly recommended to use a high-quality image. What's more important - the image's resolution should be divisible by 2, 5, and 10; for example, banners from the gallery are 1000x200 pixels.
2) Please follow these simple steps to reinstall your signature on iPhone/iPad:
Go to your Mail settings on your iPhone.
Open Signature. Click Per account. Paste the signature to each account individually.
Please do the Shake and Undo.
3) Please check the following settings on your iPhone and potentially change to resolve the issue:
Settings → Display & Brightness → Text Size → Move slider to center
Settings → Display & Brightness → View → Change it back to Standard
Settings → General → Accessibility → Larger Text → Larger Accessibility Sizes = OFF