Yes, openness reduces the effectiveness of HA noise reduction by letting the noise leak directly to your eardrums, avoiding hearing aid processing to eliminate it. Phonak and several reviewers online have noted an occlusive fit is necessary to obtain the best noise reduction results with the Spheres (or other HA noise reduction, too). Phonak has online “evidence” of the effectiveness of Spheric processing: Evidence Library | Phonak.
In the following article, labeled as a Scientific Poster, Phonak shows how users found the Sphere performed much better with Spheric Speech Clarity (SSC) than without it and better than two leading competitors. The test situation was 72 dB noise, with female speech at -3 dB compared to the noise. The fine print mentions that 27 experienced adult hearing aid users with moderate to moderately severe bilateral hearing loss aged 58 to 93 years (m=75.1±8) participated. They all wore custom acoustic couplings with 1mm vents, i.e., occlusive molds. A 1 mm vent is a small vent size, and most users with ~normal low-frequency hearing would probably find it an occlusive fit. https://www.phonak.com/content/dam/phonak/en/evidence-library/white-paper/scientific-poster/Improving_speech_understanding_and_listening_effort_for_complex_communication_environments_with_a_novel_noise_reduction_system.pdf (see the lower right of the poster for the info I reported).
YMMV. But I’ve had no problems adjusting to an occlusive fit. As I reported in another post, by increasing the Speech Enhancer setting from a default value of 14 to 16 in Target, I reduced the occlusiveness of my voice considerably. Making a joke, the sense of occlusion is all in your mind and you can get used to it by habituation. As a baby, how did you learn to accept the sound of your own voice in your head and adjust to any changes from baby sounds as you slowly grew up? Because occlusion keeps the interior of your ear canal moist, you have a greater risk of ear infections, but then folks with severe to profound hearing loss need an occlusive fit and manage to get by with it.