Here is a short reprex

valN <- 2000
valCorr <- .7
matCov <- matrix(c(1, valCorr, valCorr, 1), byrow = TRUE, ncol = 2)
vecMeans <- c(0, 0)
MASS::mvrnorm(n = valN, mu = vecMeans, Sigma = matCov) %>%
  as_tibble(.name_repair = "universal") %>%
  rename(Pre = `...1`,
         Post = `...2`) %>%
  mutate(ID = row_number(),
         group = ID %% 2) -> tibDat

First a simple plot: no unnecessary white space.

ggplot(data = tibDat,
       aes(x = Pre, y = Post)) +
  geom_point(alpha = .1) +
  geom_smooth() +
  xlim(c(-4, 4)) +
  ylim(c(-4, 4)) +
  coord_fixed()

Now facet the plot without coord_fixed(), no unwanted white space.

ggplot(data = tibDat,
       aes(x = Pre, y = Post)) +
  facet_grid(cols = vars(group),
             space = "free",
             ) +
  geom_point(alpha = .1) +
  geom_smooth() +
  xlim(c(-4, 4)) +
  ylim(c(-4, 4)) 

And then this which adds facetting and illustrates the additional white space appearing when I add coord_fixed().

ggplot(data = tibDat,
       aes(x = Pre, y = Post)) +
  facet_grid(cols = vars(group),
             space = "free",
             ) +
  geom_point(alpha = .1) +
  geom_smooth() +
  xlim(c(-4, 4)) +
  ylim(c(-4, 4)) +
  coord_fixed()

The issue is there exporting to html, pdf or word. Bigger reprex at https://www.psyctc.org/R/reprex_20240710.Rmd with and here in my Rblog. TIA for any suggestions.