Hi lakeburchell, anon2,
This is by design.
@lakeburchell
Welcome to Affinity Forums
As @anon2 suggested use a Recolour Adjustment to get the same result.
First, "images" are JPG, PNG, or TIFF files. Other formats are "documents" (PSD, PDF, EPS, SVG, .afphoto, .afdesign, etc.) and that's important to understand.
If you Place a document in Designer, you get an embedded document, which can be edited (it will have an Edit Document button on the Context Toolbar when the document is selected, or you can double-click on it to edit it.
If you place an image file, you get an embedded image which shows up as (Image) in the Layers panel. You cannot edit these embedded images separately, as you can for embedded documents. You can only edit them in place as a normal layer of your Designer document (and often will need to Rasterize them before making any changes to them).
Next, for embedded documents:
If you Place the document twice, you have two separate embedded documents, completely unrelated to each other. Changes you make while editing one of them will not affect the other.
If you Place a document, then duplicate that layer (Copy the layer and paste, or Cmd/Ctrl+J) then you have two instances of the same embedded object. If you edit one of them, the changes will apply to both.