Issue with site_url() after upgrade to 4.5.5

Upgraded my site from 4.3.8 to 4.5.5 this morning. I have gone through all the changes version by version and have the site basically running again. One issue that has me completely stopped is the behavior of site_url(). I know there were breaking changes around 4.4, but I can't for the life of me see how they apply in this case. What I have...
In my .env file:

PHP Code:

app.baseURL = 'https://domain.com/'

app.indexPage = '' 


In my view file:

PHP Code:

<a class="nav-link" href="<?= site_url() ?>" target="_blank"> 


In 4.3.8 (and before) this returned:

PHP Code:

<a class="nav-link" href="https://domain.com" target="_blank"> 


In 4.5.5 it is returning:

PHP Code:

<a class="nav-link" href="" target="_blank"> 


If I put a value in:

PHP Code:

app.indexPage = 'index' 


I get (in 4.5.5):

PHP Code:

<a class="nav-link" href="https://index/index" target="_blank"> 


If I use the example from the docs (with the original .env file settings):

PHP Code:

<a class="nav-link" href="<?= site_url('news/local/123'); ?>" target="_blank"> 


I get:

PHP Code:

<a class="nav-link" href="https://news/news/local/123" target="_blank"> 


Is there a setting somewhere I missed during the upgrades? Am I missing something obvious in the docs?
Help!